When using X-ray radiation on patients, the radiation dosages is always an important and limiting factor since the X-ray dosage always presents a risk of injury to the patient. Particularly during mammography screening, the X-ray dosage to the patient is an important factor in the evaluation of the benefit of the treatment. An accurate quality control of the X-ray equipment is therefore most important in order to be able to maintain the radiation dosages at a low level. It is normal during mammography to radiate at a relatively low acceleration voltage across the X-ray tube, for example between 25 and 30 kVp. This can lead to difficulties since the beam quality is determined by conventional technology which is adapted for considerably higher acceleration voltages. At the same, the beam quality, which for example can be determined by analysing the spectral distribution of the radiation from the X-ray tube, is a decisive factor for the radiation dosage to the breast as well as for the image quality in general.
The type of equipment which is used in order to evaluate the quality of X-ray radiation consists in most cases of many apparatuses and are generally very expensive. It would thus be desirable to be able to determine those parameters which can be suitable in order to determine the quality of X-ray and mammography equipment as a matter of routine. The parameters can for example include the appearance of an X-ray spectrum, the maximum tube voltage across the X-ray tube, the dosage velocity from the X-ray tube, the function of the X-ray tube and the X-ray generator, the resolution of the X-ray tube, the modulation transfer functions, the exposure time, etc.
The photon fluence from an X-ray tube which is used within medical radiology is very high. In order to be able to analyse photon energy spectra in a primary beam from a conventional X-ray tube, it is therefore often necessary to use very fine collimation of the beam (0.025-0.5 mm), as well as a focus-detector spacing of several meters. This implies that the measuring procedure is lengthy and time consuming.
A method for determining the appearance of an X-ray spectrum has been developed at the University of Linkoping (see SE 454 390, Matscheko and Ribberfors 1987). This can be done by measuring the dispersed radiation from a secondary distributor, a so-called Compton spectrometer, and thereafter estimating how the spectra look. This method would seem to very expensive since, on the one hand, it requires semi-conductor detectors which require cooling, for example using liquid nitrogen, and, on the other hand, advanced pulse measurement equipment. In addition, the apparatus can be said to be rather bulky because of its size and weight.
Another method for detecting X-ray radiation is known from U.S. Pat. No. 4,472,728 which describes an X-ray spectrometer based on a semi-conductor matrix for improved energy and spatial resolution. However, U.S. Pat. No. 4,472,728 shows only how the semi-conductor matrix itself is constructed and not how the actual measuring can be performed.
A further method for detecting X-ray radiation is described in SE-C2-502 298 which describes a method for determining the size and/or position of the focus of an X-ray tube. This is achieved by guiding the radiation from the X-ray tube through a slot. The image of the beam which is generated by guiding the beam through the slot is detected with the aid of linear detector elements which are arranged parallel to the slot so that the distribution of the radiation source in the transverse direction of the slot can be reproduced or measured. However, the incident radiation through the slot will be deflected and thus that radiation which is distributed in the surrounding air will also be detected. This implies, therefore, that the distributed radiation can no longer be attributed to the X-ray tube. The reason for this phenomenon is primarily because a slot is an elongate opening and contributes to reduced accuracy during measuring.